Mapping Tornado and Hail Frequency

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Mapping Tornado and Hail Frequency in the Lower 48 A Spatial Analysis of Tornado and Hail Reports from the National Climatic Data Center Tornado Report Data • The original source of the data is the Storm Prediction Center’s (SPC) Storm Data. • Through the SVRGIS project at Bal State University, the SPC data set was converted into a shapefile format compatible with the mapping software ArcGIS. • This also involved concatenating multiple path segments and removing reports with no liftoff coordinates. Tornado Report Data • Date Range: 50 year period 1957 – 2006 • Includes tornadoes of F2 or greater strength • 5,884 reports fit this criteria with no pattern of increasing activity over the reporting period. Number of Reported Tornadoes per Year 1957 - 2006 F2 and Greater 250 200 150 100 50 0 1957 1959 1961 1963 1965 1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 Tornado Report Data • Tornado reports are represented spatial y by a pair of coordinates representing touchdown and liftoff. • This implies a straight line path for al tornado reports. The Dataset Plotted: 1957 – 2006 Reports, F2 or Greater Methods for Representing and Quantifying Tornado Frequency Common Method: create a grid and count the occurrences of tornadoes within each grid cel Smal Grid Cel s: • Though some regions are more tornado prone than others, precise touchdown and liftoff locations are random. • Using smal grid cel s can result in cel s within tornado prone areas with few or no reported tornadoes. Methods for Representing and Quantifying Tornado Frequency Large Grid Cel s • Large grid cel s in effect cast a wider net and therefore are less likely to end up with “donut holes” of low or no activity within larger areas that are tornado prone. • However, the use of large cel s may over generalize frequency, and result in a more coarsely pixilated depiction of tornado frequency. • Large cel s are less sensitive to path length than smal grid cel s. Methods for Representing and Quantifying Tornado Frequency Approach of the current map: • Begin with smal grid cel : 10 x 10 mile, or 100 sq. miles. • The tornado count is taken for each cel . • These counts are used to calculate the average of each cel and it’s nearest neighboring cel s. • An interpolation technique is used to smooth the transitions between cel values. The Process up Close Reported F2 and greater Tornadoes reported in the Dal as/Ft. Worth area 1957 - 2006 The Process up Close Dal as/Ft. Worth with grid cel s color coded by tornado count